Magnetic microparticles are used as a solid phase in various applications to bind biomaterial. One advantage of microparticles is the large area of the solid phase and short diffusion lengths. The size of microparticles is generally 0.05-10 .mu.m and they are available in different materials and already activated for many applications. Magnetic particles can be moved by using a magnet.
The separation methods of magnetic particles currently used include settling a reaction vessel in a magnetic field so that particles are accumulated into a so-called pellet at the bottom of the vessel. Thereafter, the liquid which is free from particles is removed by decantation or aspiration. However, removing the liquid from the vessel must be carried out very carefully so as not to remove the particles.
Publication EP-140787 (corresponding to publication U.S. Pat. No. 4,649,116) proposes a method by which micro-size magnetic particles are separated from a liquid by using a magnetic rod which is pushed thereto. The particles are pulled off the rod by using a stronger magnet.
Publication WO-86/06493 suggests a method to be used in immunoassays, in which magnetic particles and the marked complex adhered to them are separated from a liquid by using a magnetic rod and subsequently taken to be measured. The tip of the rod comprises a fixed magnet and a removable protective cover to whose outer surface the particles adhere. It is preferable to cover the protective cover with another cover after the separation and before measuring. After the measurement, the protective covers are detached, together with the particles, and thrown away and new covers are taken for a new separation. According to the publication, the magnet can also be an electromagnet, whereby the magnetic field can be eliminated when desired.
Publication WO-87/05536 suggests a device for separating magnetic particles, comprising, on the inside, a rod movable in a vertical boring and a magnet at the lower end thereof. The device is introduced, with the magnet in the lower position, into a liquid containing particles, whereby the particles are accumulated on the end of the rod. When the magnet is allowed into the upper position, the particles can be detached from the rod. In this way, particles can be collected and transferred from one liquid into another.
However, the disclosed separation methods of magnetic particles cannot be applied very well in objects of use in which particles must be collected from a fairly large volume compared with the separation device.